In 1945 Wiman 1 showed that certain elliptic curves $E$ over $\mathbf Q$ have rank* at least 4. (It seems this was the highest known rank of an elliptic curve over $\mathbf Q$ until 1974, when Penney--Pomerance found a curve of rank at least 6.)

The method of his proof appears to be rather elementary, defining a map from $E(Q)$ to a certain abelian 2-group $A$ by using the $p$-valuations of the $x$-coordinate of a point $(x,y) \in E(\mathbf Q)$ for various primes $p$.

However, due to some combination of my inadequate German and Wiman's somewhat archaic mathematical style, I cannot decipher the exact definition of the group $A$ and the map $E(\mathbf Q) \rightarrow A$. So I ask

Question: What is the precise definition, in modern terms, of Wiman's group $A$ and map $E(\mathbf Q) \rightarrow A$?

1 Wiman, A., Über den Rang von Kurven $y^2=x(x+a)(x+b)$, Acta Math. 76, 225-251 (1945). ZBL0061.07109.

*: A bit confusingly, "rank" is used in this paper to mean "minimal number of generators of $E(\mathbf Q)$" rather than "minimal number of generators of $E(\mathbf Q)_{\operatorname{tors}}$", so to get the "correct" rank one has to subtract 2 from each of the values reported by Wiman.